Abstract:

Magneto-optical crystals of each pixel of a magneto-optical device are
magnetically and completely separated, and the entire surface thereof is
flattened.
Disclosed is a magneto-optical device including a non-magnetic substrate,
a magneto-optical crystal embedded in recessed portions formed in the
surface of the non-magnetic substrate at positions where pixels are to be
located, and a partitioning wall monolithic with the non-magnetic
substrate and magnetically separating the magneto-optical crystal from
each other at the position of a gap between the pixels, wherein the
entire surface of the magneto-optical device is flattened. A method for
manufacturing a magneto-optical device includes a digging down step,
executed at positions where pixels are to be located, of digging down
into the surface of a non-magnetic substrate in advance at the positions
where the pixels are to be formed, so that a gap portion located between
the pixels and around the dug areas remains to form a partitioning wall,
a magnetic film forming step of forming a film made of a magneto-optical
crystal over substantially the entire surface of the non-magnetic
substrate, and a surface flattening step of performing flattening by
removing a protruded portion formed by the magnetic film that has grown
on the gap portion.

Claims:

1. A magneto-optical device comprising:a non-magnetic substrate;a
magneto-optical crystal embedded in recessed portions formed in a surface
of the non-magnetic substrate at positions where pixels are to be
located; anda partitioning wall that is monolithic with the non-magnetic
substrate and that magnetically separates the magneto-optical crystal
from each other at a position of a gap between the pixels,wherein the
entire surface of the magneto-optical device is flattened.

2. The magneto-optical device of claim 1, wherein the magneto-optical
device is a magneto-optical spatial light modulator having multiple
magneto-optical crystals that serve as the pixels and that are arranged
densely in a two-dimensional array.

3. The magneto-optical device of claim 1, wherein the non-magnetic
substrate is an SGGG or a GGG single crystal substrate, and the
magneto-optical crystal is a rare-earth iron garnet single crystal.

4. The magneto-optical device of claim 2, wherein the non-magnetic
substrate is an SGGG or a GGG single crystal substrate, and the
magneto-optical crystals are rare-earth iron garnet single crystals.

5. A method for manufacturing a magneto-optical device comprising:a
digging down step, executed at positions where pixels are to be located,
of digging down into a surface of a non-magnetic substrate in advance at
the positions where the pixels are to be formed, so that a gap portion
located between the pixels and around areas that have been dug remains to
form a partitioning wall;a magnetic film forming step of forming a film
made of a magneto-optical crystal over substantially the entire surface
of the non-magnetic substrate; anda surface flattening step of performing
flattening by removing a protruded portion formed by the magnetic film
that has grown on the gap portion,wherein a plurality of the
magneto-optical crystals are embedded respectively in recessed portions
formed in the surface of the non-magnetic substrate, and the
magneto-optical crystals are magnetically separated from each other by
the partitioning wall that is monolithic with the non-magnetic substrate.

6. The method for manufacturing the magneto-optical device of claim 5,
wherein the non-magnetic substrate is an SGGG or a GGG single crystal
substrate, and the magneto-optical crystals are rare-earth iron garnet
single crystals and are formed into a film using a liquid phase epitaxial
method or a sputtering method.

7. The method for manufacturing the magneto-optical device of claim 5,
wherein coercive force of the magneto-optical crystals is reduced by
heat-treating the film at 900.degree. C. to 1,150.degree. C. in an
oxidizing atmosphere after forming the magnetic film.

8. The method for manufacturing the magneto-optical device of claim 6,
wherein the coercive force of the magneto-optical crystals is reduced by
heat-treating the film at 900.degree. C. to 1,150.degree. C. in an
oxidizing atmosphere after forming the magnetic film.

Description:

TECHNICAL FIELD

[0001]The present invention relates to a magneto-optical device and a
manufacturing method thereof that utilize the Faraday effect.

BACKGROUND ART

[0002]Magneto-optical devices utilizing magnetic films include an optical
isolator, an optical switch and the like in the optical communication
field and a magneto-optical spatial light modulator (MOSLM) and the like
in the optical information processing field. A magneto-optical spatial
light modulator is a magneto-optical device that spatially modulates the
amplitude, the phase, and the state of polarization of a light beam by
utilizing the Faraday effect of a magnetic film, and the modulator is
recently expected to be applied to hologram recording, various displays,
etc.

[0003]To parallel-process a light beam, the above magneto-optical spatial
light modulator is configured such that multiple pixels (cells) of which
the direction of magnetization of each magnetic film can be independently
controlled are arranged in a two-dimensional array. The operation of each
of the pixels will be described referring to FIG. 5. An incident light
beam that has been plane-polarized after passing through a first
polarizer 10 enters each pixel 12 of the magneto-optical spatial light
modulator. The incident light beam passes through a transparent substrate
14 such as an SGGG (Substituted Gadolinium Gallium Garnet) substrate and
a magnetic film 16, is reflected on a metal film 18, again passes through
the magnetic film 16 and the transparent substrate 14, and exits out. At
this point, due to the Faraday effect of the magnetic film 16, the
direction of the polarization of the light beam returning back after
passing through each pixel 12 and being reflected is rotated by a
predetermined angle. In this case, assuming that a Faraday rotation of
+θF (for example, +45°) is generated when a magnetic
field (+H) in the positive direction is applied to a pixel in the upper
row, a Faraday rotation of -θF (for example, -45°) is
generated when a magnetic field (-H) in the negative direction is applied
to a pixel in the lower row. These reflected light beams reach a second
polarizer 20. When the polarizing transmitting face of the second
polarizer is set at +45°, the light beam in the upper row that has
been Faraday-rotated by +45° passes through the second polarizer
20 (the light is "ON"), but the light beam in the lower row that has been
Faraday-rotated by -45° is blocked by the second polarizer 20 (the
light is "OFF"). In this manner, "ON" and "OFF" of the reflected beam by
each pixel can be controlled by controlling the direction of the magnetic
field applied to each pixel.

[0004]In the magneto-optical spatial light modulator, each pixel is not an
individual device that is completely independent as a pixel. In practice,
an integrated structure is employed that is manufactured by growing a
magnetic film over the entire surface of a substrate using the LPE
(Liquid Phase Epitaxy) method, etc., and magnetically partitioning the
magnetic film into multiple pixels. This is because each pixel needs to
be very small and to be arranged accurately and densely. Therefore, a
structure needs to be employed that an arbitrary pixel does not influence
other adjacent pixels in terms of flux reversal of each pixel.

[0005]A method of digging a gap at a position between the pixels of the
magnetic film formed on the substrate surface is common as the method of
separating each pixel securely and magnetically. More specifically, a
groove is formed by dry etching or wet etching as the gap. However, such
a separating structure has caused a significant problem in terms that it
will become difficult to achieve multi-layering (it will become difficult
to wire driving lines) when this device is used as a magneto-optical
spatial light modulator because unevenness is generated on the surface.
That is, this unevenness may increase the resistance value of the driving
line, and in an extreme case, disconnection may occur.

[0006]To flatten such an uneven surface, covering the surface with a
flattening material such as a polymer can be considered. However, this
kind of flattening material is hard to be employed because the material
shrinks by heat when it is baked, and therefore, the magnetic property of
the pixels may be varied (more specifically, the coercive force may be
increased).

[0007]In addition, for example, U.S. Pat. No. 5,473,466 discloses a
technique that: a film pattern that can be oxidized is formed with, for
example, silicon (Si) at the position of each pixel on a magnetic garnet
material; the magnetic garnet material just beneath the Si film is
reduced and transformed by heat-treating the entire work; and, thereby,
flux reversal is enabled for each pixel. However, when the entire
magnetic garnet material is heat-treated using a film that can be
oxidized such as Si, the periphery of the Si film is also reduced due to
thermal diffusion. Therefore, the outline of each pixel becomes vague and
size variation of the pixel is also caused. Therefore, the distance
between pixels must be made long. As the gap length between pixels
becomes longer, the amount of information per unit area is reduced.
Therefore, this device is not suitable for a use that processes a large
amount of information at a high speed.

DISCLOSURE OF INVENTION

[0008]The object of the present invention is to provide a magneto-optical
device that is structured such that a magneto-optical crystal of each
pixel is completely separated magnetically, and the entire surface
thereof is flat, and a manufacturing method thereof.

[0009]An aspect of the present invention provides a magneto-optical device
including: a non-magnetic substrate; a magneto-optical crystal embedded
in recessed portions formed in a surface of the non-magnetic substrate at
positions where pixels are to be located; and a partitioning wall that is
monolithic with the non-magnetic substrate and that magnetically
separates the magneto-optical crystal from each other at a position of a
gap between the pixels, wherein the entire surface of the magneto-optical
device is flattened.

[0010]The magneto-optical device is, for example, a magneto-optical
spatial light modulator having multiple magneto-optical crystals, which
serve as the pixels, arranged densely therein in a two-dimensional array.
An SGGG or a GGG (Gadolinium Gallium Garnet) single crystal substrate can
be used as the non-magnetic substrate. A rare-earth iron garnet single
crystal can be typically employed as the magneto-optical crystal.

[0011]Another aspect of the present invention provides a manufacturing
method of a magneto-optical device, including: a digging down step,
executed at positions where pixels are to be located, of digging down
into a surface of a non-magnetic substrate in advance at the positions
where the pixels are to be formed, so that a gap portion located between
the pixels and around areas that have been dug remains to form a
partitioning wall; a magnetic film forming step of forming a film made of
a magneto-optical crystal over substantially the entire surface of the
non-magnetic substrate; and a surface flattening step of performing
flattening by removing a protruded portion formed by the magnetic film
that has grown on the gap portion, wherein a plurality of the
magneto-optical crystals are embedded respectively in recessed portions
formed in the surface of the non-magnetic substrate, and the
magneto-optical crystals are magnetically separated from each other by
the partitioning wall that is monolithic with the non-magnetic substrate.

[0012]For example, an SGGG or a GGG single crystal substrate is used as
the non-magnetic substrate; a rare-earth iron garnet single crystal is
used as the magneto-optical crystal; and the film can be formed using the
liquid phase epitaxial method or the sputtering method. After the
magnetic film has been epitaxial-grown, the film is heat-treated at
900° C. to 1,150° C. in an oxidizing atmosphere, and
thereby, the coercive force can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is an explanatory diagram of an example of a magneto-optical
device according to the present invention;

[0014]FIGS. 2A to 2F are explanatory diagrams of an example of
manufacturing steps of the magneto-optical device according to the
present invention;

[0015]FIGS. 3A and 3B depict SEM photos of the surface of a prototype;

[0016]FIG. 4 depicts a graph showing a variation of the magnetic property
caused by a heat treatment; and

[0017]FIG. 5 is an explanatory diagram of operations of a magneto-optical
spatial light modulator.

DESCRIPTION OF THE REFERENCE NUMERALS

[0018]30 non-magnetic substrate

[0019]32 recessed portion

[0020]34 magneto-optical crystal

[0021]36 partitioning wall

DETAILED DESCRIPTION OF THE INVENTION

[0022]FIG. 1 is an explanatory diagram of an example of a magneto-optical
device according to the present invention. The invention is structured
such that: each magneto-optical crystal 34 is embedded in each recessed
portion 32 formed at each pixel position on the surface of a non-magnetic
substrate 30; the magneto-optical crystals 34 are magnetically separated
by a partitioning wall 36 that is monolithic with the non-magnetic
substrate at the position of a gap between the pixels; and the entire
surface is flattened. The magneto-optical crystals 34, which are
independent from each other, serve as pixels (cells) respectively.

[0023]The above structure can be manufactured by undergoing the three
steps of: a digging down step, executed at positions where pixels are to
be located, of digging down into the surface of the non-magnetic
substrate (for example, an SGGG or a GGG single crystal substrate) in
advance at the positions where the pixels are to be formed to a depth
deeper than the thickness of a necessary magnetic film (magneto-optical
crystal), so that a gap portion between the pixels and around the dug
areas remains to form a partitioning wall; a magnetic film forming step
of forming a film made of a magneto-optical crystal (for example, a
rare-earth iron garnet single crystal) over substantially the entire
surface of the non-magnetic substrate; and a surface flattening step of
performing flattening by removing protruded portions formed by the
magnetic film that has grown on the gap portions.

[0024]The non-magnetic substrate at the pixel positions is dug down in
advance more deeply than the thickness of the magnetic film to be
epitaxial-grown. By liquid-phase-epitaxial growing the film on the
surface of the non-magnetic substrate having the above shape, the
structure can be manufactured such that the partitioning wall which is
monolithic with the non-magnetic substrate is formed in the gap portion
between the pixels. At this point, the magnetic film is also grown at the
position of the gap on the non-magnetic substrate and, after growth, a
protrusion structure similar to that of the underlying non-magnetic
substrate is generated. However, this protruded portion can be removed by
etching or polishing after flattening with a flattening material.
Therefore, the magnetic films (the magneto-optical crystals) at the pixel
positions are finally completely separated magnetically and a
two-dimensional magnetic pixel array with a flat surface can be
manufactured. Even when the etching is executed after flattening with the
flattening material, the flattening material is finally removed and,
therefore, the flattening material does not adversely affect the magnetic
property of the pixels.

[0025]FIGS. 2A to 2F depict an example of manufacturing steps of a
magneto-optical device according to the present invention. This is an
example of an application to the manufacture of a magneto-optical spatial
light modulator, which includes the following process steps.

[0026]FIG. 2A: Patterning of a tetragonal lattice is executed on an SGGG
substrate 40 of which the surface is in (111) orientation with
photoresists 42 using the photolithography technique. Multiple
rectangular micro areas to which no photoresist is adhered are
respectively the positions where the pixels are to be formed.

[0027]FIG. 2B: Multiple rectangular recessed portions 44 are dug using ion
milling such that a step of 1.0 μm is formed in the SGGG substrate 40
at each of the positions where the pixels are to be formed.

[0028]FIG. 2C: The photoresists remaining on the surface are removed.
Thereby, it is possible to obtain a SGGG substrate 40 having a surface
structure in which the positions where the pixels are to be formed are
dug by 1.0 μm and, in contrast, the positions of the gaps between
pixels are protruded by 1.0 μm. The protruded portion at the position
of the gap between the pixels becomes a partitioning wall 48.

[0029]FIG. 2D: On the SGGG substrate 40 which the surface thereof has been
processed as above, a Bi-replaced iron garnet thin film 50 is grown using
the liquid phase epitaxial method. The film thickness to be grown is
assumed to be 3.0 μm. The surface at this time shows a structure that
has protruded portions 51 similar to those of the underlying SGGG
substrate, and the step height of each portion 51 is 1.0 μm.

[0030]FIG. 2E: Thereafter, by polishing the surface, the protruded
portions that have appeared on the magnetic film are shaved, and a flat
surface is produced.

[0031]FIG. 2F: The entire surface is shaved using ion milling, and this
shaving is stopped when the magnetic film at the position of the gap is
removed. Thereby, the Bi-replaced iron garnet single crystals 52 that are
to serve as the pixels are embedded in the recessed portions 44 of the
SGGG substrate 40, the partitioning wall 48 that is monolithic with the
substrate is provided at the position of the gap between the pixels, and,
thereby, the complete magnetic separation is achieved between the pixels.

[0032]FIGS. 3A and 3B depict SEM photos of the surface of a prototype.
FIG. 3A depicts the state immediately after the magnetic film has been
grown. The unevenness of the underlying substrate reflects on the surface
of the magnetic film, and a step of 1 μm has appeared. In contrast,
FIG. 3B depicts the state where the magnetic film above the position of
the gap is completely removed by polishing and etching. It can be seen
from the photos that the surface is flattened.

[0033]The Bi-replaced iron garnet single crystal thin film in the state
where the film has been formed using the liquid phase epitaxial method
has a coercive force that is too strong (>40,000 [A/m]) when no
additional process is applied to the film, and a large driving current is
necessary for the flux reversal of the pixels. Therefore, after the
magnetic film has been epitaxial-grown (after the surface has been
flattened in the step of FIG. 2F), the film is heat-treated at
900° C. to 1,150° C. in an oxidizing atmosphere. Thereby,
the magnetic field necessary for the flux reversal of each pixel can be
reduced, and the driving current can also be reduced. FIG. 4 depicts the
state where the reversed magnetic field of the pixels is reduced by the
heat-treatment in the atmosphere.